1. Field of the Invention
The invention relates to the field of determining parameters of particles, droplets or the like employing laser light scattering.
2. Prior Art
A number of techniques employing laser light scattering are well-known for determining both the size and velocity of particles. In one system, commonly referred to as a particle sizing interferometer, a pair of laser beams of equal intensity are caused to cross at the sample volume. An interference pattern between the beams is established by this crossing. Particles moving through this pattern scatter light in proportion to the spatially varying light intensity within the pattern. From this scattering, the information concerning both the particles' size and velocity can be computed. The particles' size is determined from the visibility. A more detailed explanation of this interferometer is given in "On-Line Particle Monitoring Instruments", by Bachalo, Geffken and Weth, 1978 Symposium on Instrumentation and Control for Fossil Demonstration Plants, June 19-21, 1978.
Significant for purposes of this patent is the fact that in these interferometers, the collection means used for determining particle sizing is always in-line or on-axis with the pair of laser beams which provide the interference pattern. It has been understood in the art that such on-axis collection for particle sizing was necessary. For example, see "The Interferometric Observations of Dynamic Particle Size, Velocity and Number Density", a dissertation presented to the Graduate Council of the University of Tennessee by W. Michael Farmer, March, 1973, at page 124.
There has long been a need to measure droplet size in sprays or the like. Such measurements are useful in aircraft icing studies, such as at engine inlets and other icing applications, weather studies (e.g., fog), fuel sprays in combustion applications, numerous nozzle sprays, etc. These measurements are also useful in determining the size of gas bubbles in liquids. The sizing techniques employed in the interferometer are difficult to use in these applications since the density of droplets is high when compared to typical particle densities found in flue gasses, etc. The sensed information from the interference pattern may represent scattering associated with multiple droplets. This information cannot readily be processed to determine particle size with this otherwise reliable system. The particle sizing interferometer is also limited in the size range (2 .mu.m to 200 .mu.m) over which it can be used.
The present invention discloses two techniques which permit particle sizing using the interferometric method. In the presently preferred embodiment, off-axis collection is employed to reduce the probe volume associated with the collection lens and to select the scattering phenonmena used to size the droplets. While off-axis collection has been employed for interferometer velocity determinations, as mentioned, it has been thought that such collection was not possible for sizing determinations. The problem with the prior art analysis, which led to the conclusion that the collection must be on-axis, is described.
In an alternate embodiment, the probe volume is "sharpened" by employing a second laser beam in addition to the pair of beams used to establish the interference pattern. As will be described, the second beam acts as a "pointer" to provide better fringe control. This results in more accurate parameter determinations. The use of the pointer beam also has application in light scattering instruments other than the interferometer, as will be discussed.